The wettability of a solid surface is commonly measured by contact angle (CA). The contact angle of a liquid on a flat surface is the result of three different types of surface tension at the solid/liquid/gas interface, and is measured by Young's equation. Hydrophilicity of a solid surface refers to a contact angle less than 90° while hydrophobicity refers to a contact angle higher than 90°. If the contact angle is less than 10°, the surface is often designated as hydrophilic, provided that the surface do not react or dissolve in the water.
Paints and coatings, while protecting the substrate from external environment, themselves get dirty. The dirt dulls the coating by increasing the light scattering or by modifying the color of the coating. The dirt also affects the durability of a paint. A hydrophilic surface would allow water to spread out in a thin pool, sweeping dirt off the surface as the water puddles or thins out and trickles away. This type of “self-cleaning” behavior is advantageous to an exterior paint, as it keeps the coating clean without requiring extensive cleaning.